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Under certain conditions, a quote can reserve an invoice number. When the customer comes back a bit later and finalizes the order, you will see an older number pop up later in the invoice order sequence.
I pretty much ignore it and let it take the orders. There are some discrepancies you’ll note with invoice number assignment. You’ve just caught one. The other is when it drops the invoice number on declined cards, possibly guest quotes. Many people ask about this, but Varien/Magento seems to think it is normal operation.